Etching method and etching composition useful for the method

ABSTRACT

In etching of silicon nitride with a phosphorus type, if etching is carried for a long time, silicon oxide tends to precipitate, and it has been impossible to constantly carry out the etching for a long period of time. 
     By an etching method for silicon nitride using a composition comprising a phosphorus compound, a boron compound, a silicon compound and/or their fluorides thereof, there will be no precipitation of silicon oxide even when the composition is used for a long time. It is particularly preferred to further add nitric acid and/or a nitrate, whereby stability of selectivity will be increased.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an etching composition for siliconnitride. More particularly, it relates to a composition capable ofetching silicon nitride to be used for an insulating film for e.g.semiconductor devices or flat panel displays.

2. Discussion of Background

Silicon nitride is a very important compound as a ceramic material or amaterial for semiconductors. In a process for producing semiconductors,there is a step wherein it is required to selectively etch only siliconnitride without presenting any damage to silicon oxide. At present, inthis step, high purity phosphoric acid is mainly used. However, highpurity phosphoric acid has had a problem that when it is used at a hightemperature of at least 150° C., damage to silicon oxide tends to besubstantial.

As a means to suppress such damage to silicon oxide, high purityphosphoric acid having silicon dissolved therein or phosphoric acidhaving hexafluorosilicic acid added thereto, has been proposed (PatentDocuments 1 to 3). However, hexafluorosilicic acid is a volatilesubstance, and at the operation temperature of at least 150° C.,hexafluorosilicic acid tends to be volatile, whereby no constant effectof its addition is obtainable, and continuous use of such an etchant hasbeen difficult. Further, in a case where hexafluorosilicic acid isadded, precipitation of an insoluble silicon compound from the etchanttends to be accelerated, and it has been problematic to use it in anindustrial operation.

On the other hand, in a wet etching device for semiconductors, etchingis carried out while removing foreign particles, etc. in the etchant bysubjecting the etchant to filtration and recycling to keep it clean.Also in etching of silicon nitride with a heated phosphoric acid aqueoussolution, a method of removing precipitates by similar filtration andrecycling, has been proposed (Patent Document 4). However, there hasbeen a problem that such a precipitated silicon compound tends to adhereto semiconductor wafers, etc., before being filtered off.

As a method for etching silicon nitride at a temperature of not higherthan 100° C., a method of adding fluorosilicic acid or a fluorosilicateto an etchant comprising phosphoric acid, hydrofluoric acid and nitricacid, is disclosed. However, with an etching composition having bothhydrofluoric acid and nitric acid added to phosphoric acid, the damageto silicon oxide as another semiconductor material tends to be large,and it is problematic to use such a method for a semiconductor process,and such an adverse effect has been more remarkable when it is used at ahigh temperature.

Thus, heretofore, there has been no etching composition which is capableof selectively etching silicon nitride at a high temperature constantlyfor a long period of time.

Patent Document 1: JP-A-6-349808

Patent Document 2: JP-A-2000-133631

Patent Document 3: JP-A-8-64574

Patent Document 4: JP-B-3-20895

SUMMARY OF THE INVENTION

In view of the above described problems, it is an object of the presentinvention to provide an etching method for silicon nitride, wherebyfrequency of replacing a phosphoric acid etchant can be reduced, i.e. aphosphoric acid etchant can be constantly used over a long period oftime and whereby the damage to silicon oxide by high purity phosphoricacid and precipitation are prevented.

The present inventors have conducted an extensive study with respect toetching of silicon nitride and as a result, have found that with anetching composition comprising a phosphorus compound and/or a fluoridethereof, a boron compound and/or a fluoride thereof, a silicon compoundand/or a fluoride thereof, and water, it is possible to prevent thedamage and precipitation of silicon oxide better than a conventionalphosphoric acid etchant, and silicon nitride can be removed by etchingconstantly for a long period of time. The present invention has beenaccomplished on the basis of this discovery.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Now, the present invention will be described in further detail.

The etching method for silicon nitride of the present invention employsa composition comprising a phosphorus compound and/or a fluoridethereof, a boron compound and/or a fluoride thereof, a silicon compoundand/or a fluoride thereof, and water.

The phosphorus compound and/or a fluoride thereof to be used in theetching method of the present invention is phosphoric acid or phosphorusfluoride. The phosphoric acid or phosphorus fluoride to be used, is notparticularly limited, and one commonly available, may be used.

The phosphorus fluoride to be used in the etching method of the presentinvention is preferably at least one member selected from the groupconsisting of phosphorus trifluoride, phosphorus pentafluoride, HPF₆, afluorophosphoric acid such as HPO₂F₂ or H₂PO₃F, and a fluorophosphatesuch as ammonium fluorophosphate.

The boron compound and/or a fluoride thereof to be used in the etchingmethod of the present invention is preferably at least one memberselected from the group consisting of boric acid, a borate such asammonium borate, boron trifluoride, HBF₄, a fluoroboric acid such asHBF₃(OH), HBF₂(OH)₂ or HBF(OH)₃, and a fluoroborate such as ammoniumfluoroborate.

The etching method of the present invention preferably employs acomposition comprising phosphoric acid, from 0.001 to 10 wt % of a boroncompound and/or phosphorus fluoride and from 1 to 30 wt % of water,particularly preferably a composition comprising from 0.001 to 1 wt % ofa boron compound and from 15 to 30 wt % of water.

If the boron compound and/or phosphorus fluoride is less than 0.001 wt%, the effect to prevent precipitation of silicon oxide tends to besmall, and if more than 10 wt % of the boron compound and/or phosphorusfluoride is incorporated, the material which should not be etched, suchas a silicon oxide film, is likely to be etched. Further, if water isless than 1 wt %, the etching rate of silicon nitride tends to be low,and if water exceeds 30 wt %, the boiling point of the etchingcomposition tends to be low, and it tends to be difficult to maintainthe temperature suitable for etching. Industrially, the boron compoundand/or phosphorus fluoride is particularly preferably within a range offrom 0.01 to 0.2 wt %.

The boron content in the composition to be used in the etching method ofthe present invention is preferably at least 0.014 wt %, more preferablyfrom 0.014 wt % to 10 wt %, particularly preferably from 0.014 wt % to 5wt %, from the viewpoint of the etching rate of silicon nitride,prevention of precipitation of silicon oxide and stability ofselectivity.

The silicon compound and/or a fluoride thereof in the composition to beused in the etching method of the present invention is particularlypreferably silicic acid, a silicate, hexafluorosilicic acid, ahexafluorosilicate, an alkoxysilane such as tetraethoxysilane ortetramethoxysilane, or an alkylsilane such as methyltrimethoxysilane,but a soluble silicon compound other than these may also be used. As thesilicate or the hexafluorosilicate, an ammonium salt is preferred.Silicic acid or a silicate may be one obtained by adding silicon oxideor the like to sulfuric acid or phosphoric acid, followed by heating fordissolution. Hexafluorosilicic acid or a hexafluorosilicate may be onewhich is industrially available, or one obtained by reacting silicicacid with hydrofluoric acid and further converting it to its salt. Bythe addition of such a silicon compound, it is possible to reduce thedamage to a silicon oxide film.

The concentration of a silicon compound in the etching method of thepresent invention is preferably from 0 to 0.5 wt %, particularlypreferably from 0.001 to 0.1 wt %. If it is added more than 0.5 wt %,silicon oxide is likely to precipitate.

The silicon content in the composition in the etching method of thepresent invention is particularly preferably from 0.001 to 0.01 wt %,and the fluorine content is particularly preferably from 0.010 to 0.050wt %. If the silicon content is less than 0.001 wt %, the etching rateof silicon oxide at the initial stage of batch is high, and theselectivity of the initial batch is low and tends to be hardlystabilized. On the other hand, if it is more than 0.01 wt %, the etchingrate of silicon nitride will be low as the batch treatment is repeated,and the selectivity tends to be hardly stabilized.

By the etching method of the present invention, high selectivity isobtainable from the initial stage of batch, and the stability ismaintained over a long period of time. The reason as to why suchcharacteristics are obtainable is not necessarily clearly understood,but it is considered that in the etching composition, a compositecompound of boron, silicon and fluorine (hydrofluoric acid) is formed,whereby an extremely stable state is maintained.

In the etching method of the present invention, it is preferred to addnitric acid and/or a nitrate. By the addition of nitric acid and/or anitrate, it is possible to reduce the damage to a silicon oxide film.Particularly when nitric acid and/or a nitrate is used in combinationwith a phosphorus compound and/or a fluoride thereof, a boric compoundand/or a fluoride thereof, even when batch treatment is repeated,fluctuation of the selective etching of silicon nitride to silicon oxideis small, and the stability tends to be very high. As the nitrate, it ispreferred to use ammonium nitrate. The concentration of nitric acidand/or a nitrate in the etching method of the present invention ispreferably from 0 to 20 wt %, particularly from 0.0001 to 10 wt %,further preferably from 0.01 to 1 wt %, especially preferably from 0.01to 0.2 wt %. On the other hand, if it exceeds 20 wt %, the etching rateof silicon nitride tends to decrease.

The etching method of the present invention exhibits an excellentperformance when used for etching of silicon nitride, particularly foretching of silicon nitride to be used as an insulating film forsemiconductor devices or flat panel displays. In a semiconductor device,silicon nitride is deposited to form a film on a semiconductor substrateby e.g. a CVD method (a chemical vapor phase deposition method), but toform an element or circuit, it is necessary to remove unnecessaryportions by etching. By using the etching method of the presentinvention, it is possible to selectively etch silicon nitride constantlyfor a long period of time.

The temperature for the etching method of the present invention is from120 to 180° C., preferably from 130 to 170° C. If the temperatureexceeds 180° C., a semiconductor material other than silicon nitride islikely to be damaged, and if the temperature is lower than 120° C., ittends to be difficult to carry out etching of silicon nitride at anindustrially satisfactory rate. In the etching method of the presentinvention, as etching of silicon nitride proceeds, the siliconconcentration in the etchant becomes high. In order to is avoid such aphenomenon, a phosphorus compound and/or a fluoride thereof, a boroncompound and/or a fluoride thereof may additionally be added. The boroncompound or phosphorus fluoride may be added alone, or may be added asan etching composition of the present invention containing a boroncompound and/or phosphorus fluoride.

Further, in a case where batch treatment is repeated, some components ofthe etching composition of the present invention will be consumed orevaporated, whereby the effects of the present invention may decrease.Therefore, the composition may be used while optionally adding at leastone member selected from the group consisting of a phosphorus compoundand/or a fluoride thereof, a boron compound and/or a fluoride thereof, asilicon compound and/or a fluoride thereof, nitric acid and/or anitrate, and water in the etching composition of the present invention.

In the etching method of the present invention, the etching may beaccelerated by using e.g. ultrasonic waves at the time of etchingsilicon nitride.

EFFECTS OF THE INVENTION

According to the etching method of the present invention, selectivityfor etching of silicon nitride is high, there is no excess dissolutionor precipitation of silicon oxide during etching for a long period oftime, and silicon nitride can be selectively etched, whereby theproductivity is high.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the etching rates of silicon nitride under theconditions of Examples 11 to 13.

FIG. 2 is a graph showing the etching rates of silicon oxide under theconditions of Examples 11 to 13.

FIG. 3 is a graph showing the stability of the etching rates of siliconnitride under the conditions of Examples 11 to 13.

FIG. 4 is a graph showing the selectivities (silicon nitride/siliconoxide) under the conditions of Examples 11 to 13.

FIG. 5 is a graph showing the etching rates of silicon nitride under theconditions of Comparative Examples 2 and 3.

FIG. 6 is a graph showing the etching rates of silicon oxide under theconditions of Comparative Examples 2 and 3. (Negative etching ratesindicate precipitation of silicon oxide.)

FIG. 7 is a graph showing the etching rates of silicon nitride under theconditions of Examples 9 and 10.

FIG. 8 is a graph showing the etching rates of silicon oxide under theconditions of Examples 9 and 10.

Now, the present invention will be described in further detail withreference to Examples, but it should be understood that the presentinvention is by no means restricted thereto. In the following Examples,“%” means “wt %”

Example 1

A silicon wafer (15 mm square) having SiN deposited in a thickness of300 nm by a CVD method and a silicon wafer (15 mm square) having athermally oxidized film formed in a thickness of 1,000 nm were immersedat 150° C. for 30 minutes in 100 g of an etching composition comprising0.02% of fluoroboric acid, 0.002% of ammonia, 5% of water and the restbeing phosphoric acid. The wafers were taken out, washed with water anddried, and then the film thicknesses of SiN and the thermally oxidizedfilm were measured by an optical interferotype film thickness meter.This operation was regarded as 1 batch and was continuously repeated for14 batches. In the 14th batch, the SiN etching rate was 6.10 nm/min, andthe etching rate of the thermally oxidized film was 0.03 nm/min. Even inthe 14th batch, no precipitation of silicon oxide was observed on thethermally oxidized film, and the thermally oxidized film was found to beslightly etched.

This etching composition initially started in a state where it containedno soluble silicic acid, but immediately after the initiation of itsuse, the composition became a state where it contained soluble silicicacid. The etching selectivity for silicon nitride to silicon oxide(silicon nitride/silicon oxide) in each batch was examined, whereby itwas found to be improved from 18 in the first batch to 130 in the 14thbatch.

Example 2

Etching of silicon nitride was carried out under the same conditions asin Example 1 by using a composition having 0.03% of hexafluorosilicicacid further added to the composition in Example 1.

Even in the 14th batch, no precipitation of silicon oxide was observed.

The selectivity for silicon nitride to silicon oxide was 220 from thefirst batch and thus was high from the beginning.

Example 3

Etching of silicon nitride was carried out under the same conditions asin Example 1 by using a composition wherein 0.02% of fluoroboric acid inthe composition in Example 1 was changed to 0.02% of fluorophosphoricacid.

In the 14th batch, the SiN etching rate was 6.00 nm/min, and the etchingrate of the thermally oxidized film was 0.06 nm/min. Even in the 14thbatch, no precipitation of silicon oxide was observed on the thermallyoxidized film, and the thermally oxidized film was found to be slightlyetched.

This etching composition initially started in a state where it containedno soluble silicic acid, but immediately after the initiation of itsuse, the composition was in a state where it contained soluble silicicacid. The etching selectivity for silicon nitride to silicon oxide(silicon nitride/silicon oxide) in each batch was examined, whereby itwas found to be 15 in the first batch but 100 in the 14th batch.

Example 4

Etching of silicon nitride was carried out under the same conditions asin Example 1 by using a composition wherein in the composition inExample 1, 0.02% of fluoroboric acid was changed to 0.01% of fluoroboricacid and 0.01% of fluorophosphoric acid.

In the 14th batch, the SiN etching rate was 6.00 nm/min, and the etchingrate of the thermally oxidized film was 0.05 nm/min. Even in the 14thbatch, no precipitation of silicon oxide was observed on the thermallyoxidized film, and the thermally oxidized film was found to be slightlyetched.

The etching composition initially started in a state where it containedno soluble silicic acid, but immediately after the initiation of itsuse, the composition was in a state where it contained soluble silicicacid. The etching selectivity for silicon nitride to silicon oxide(silicon nitride/silicon oxide) in each batch was examined, whereby itwas 16 in the first batch, but 120 in the 14th batch.

Example 5

Etching of silicon nitride was carried out under the same conditions asin Example 1 by using a composition wherein to the composition inExample 1, 0.03% of hexafluorosilicic acid was further added.

0.02% of fluoroboric acid was added in an amount of 4% to every batch,and even in the 14th batch, no precipitation of silicon oxide wasobserved.

The selectivity for silicon nitride to silicon oxide was 200 from thefirst batch and thus was high from the beginning.

Example 6

Etching of silicon nitride was carried out under the same conditions asin Example 5 by using a composition wherein in the composition inExample 5, 0.02% of fluoroboric acid was changed to 0.05% of fluoroboricacid, and 0.1% of nitric acid was further added. 0.05% fluoroboric acidwas added in an amount of 4% to every batch, and on the first batch, SiNetching rate was 5.42 nm/min, and in the 12th batch, the SiN etchingrate was 5.00 nm/min, and the etching rate of the thermally oxidizedfilm was 0.01 nm/min. Even in the 12th batch, no precipitation ofsilicon oxide was observed on the thermally oxidized film, and thethermally oxidized film was found slightly etched.

The etching selectivity for silicon nitride to silicon oxide (siliconnitride/silicon oxide) in each batch was examined, whereby theselectivity was 325 in the first batch and thus was high from thebeginning, and it was 440 in the 12th batch and thus was maintained at ahigh level.

Example 7

Etching of silicon nitride was carried out under the same conditions asin Example 5 by using a composition wherein in the composition inExample 6, 0.1% of nitric acid was changed to 1% of nitric acid.

In the first batch, the SiN etching rate was 5.57 nm/min, and in the12th batch, the SiN etching rate was 5.02 nm/min, and the etching rateof the thermally oxidized film was 0.02 nm/min. Even in the 12th batch,no precipitation of silicon oxide was found in the thermally oxidizedfilm, and the thermally oxidized film was found to be slightly etched.

The etching selectivity for silicon nitride to silicon oxide (siliconnitride/silicon oxide) in each batch was examined, whereby it was 220 inthe first batch and 328 in the 12th batch.

Example 8

Etching of silicon nitride was carried out under the same conditions asin Example 1 by using a composition wherein in the composition inExample 1, 0.02% of fluoroboric acid was changed to 0.05% of fluoroboricacid, and 0.1% of ammonium nitrate was further added.

In the first batch, the SiN etching rate was 5.44 nm/min, and in the12th batch, the SiN etching rate was 4.99 nm/min, and the etching rateof the thermally oxidized film was 0.01 nm/min. Even in the 12th batch,no precipitation of silicon oxide was observed on the thermally oxidizedfilm, and the thermally oxidized film was found to be slightly etched.

The etching composition initially started in a state where it containedno soluble silicic acid, but immediately after the initiation of itsuse, the composition was in a state where it contained soluble silicicacid. The etching selectivity for silicon nitride to silicon oxide(silicon nitride/silicon oxide) in each batch was examined, whereby itwas 340 in the first batch and thus was high from the beginning, and itwas 441 in the 12th batch.

Example 9

Etching was carried out in the same manner as in Example 1 except thatto the etching composition comprising 0.03% of hexafluorosilicic acid,2.5% of water and the rest being phosphoric acid, 0.05% fluoroboric acidwas added to every batch in an amount of 4% per batch.

Even when the batch was repeated, there was no decrease in the etchingrate of silicon nitride, and etching of silicon oxide was alsomaintained to be at a very low level.

Example 10

Etching was carried out in the same manner as in Example 1 except thatto the composition in Example 9, 0.1% (1,000 ppm) of nitric acid wasfurther added.

Like in Example 9, there was no decrease in the etching rate of siliconnitride, and etching of silicon oxide was also maintained to be at avery low level, but particularly after the batch was repeated, theselectivity was higher than in Example 9.

TABLE 1 Selectivity Example 9 Example 10 Number of batches (Absence ofHNO₃) (Presence of HNO₃) 1 297 325 2 197 371 3 325 519 4 338 416 5 189242 8 150 354 10 144 206 11 106 376 12 118 439

Example 11

Etching was carried out in the same manner as in Example 1 except thatto an etching composition comprising 0.0043% of silicon, 0.0174% offluorine, 0.015% of boron (corresponding to 0.022% of hexafluorosilicicacid and 0.086% of boric acid), 3.75% of water and the rest beingphosphoric acid, 0.05% fluoroboric acid was added to every batch in anamount of 4.5% per batch, and evaluation was carried out at 155° C.

By this composition, a constant selectivity was obtained from the firstbatch, and there was no decrease in the etching rate of silicon nitrideeven when the is batch was repeated.

Example 12

Etching of silicon nitride was carried out under the same conditions asin Example 11 except that silicon was 0.0039% and fluorine was 0.0158%(corresponding to 0.02% of hexafluorosilicic acid).

There was no decrease in the etching rate of silicon nitride even whenthe batch was repeated, but the etching rate of the thermally oxidizedfilm in the first and second batches was at least 0.05 nm/min and thuswas larger than in Example 11.

Example 13

Etching of silicon nitride was carried out under the same conditions asin Example 11 except that silicon was 0.0049%, and fluorine was 0.0198%(corresponding to 0.025% of hexafluorosilicic acid).

Although etching of silicon oxide was maintained to be at a low level, adecrease in the etching rate of silicon nitride was observed up to thethird batch.

Comparative Example 1

Etching was carried out in the same manner as in Example 1 except thatneither fluoroboric acid nor fluorophosphoric acid was added. In the14th batch, the SiN etching rate was 2.49 nm/min. In the 14th batch,precipitation of silicon oxide was observed on the thermally oxidizedfilm, and the thickness of the thermally oxidized film was found to beincreased.

Comparative Example 2

Etching was carried out in the same manner as in Example 1 by using anetching composition comprising 0.03% of hexafluorosilicic acid, 2.5% ofwater and the rest being phosphoric acid.

As the batch was repeated, the etching rate of silicon nitridedecreased.

Comparative Example 3

To the composition in Comparative Example 2, 0.1% hexafluorosilicic acidwas sequentially added to every batch in an amount of 4% per batch.

Even when the batch was repeated, there was no decrease in the etchingrate of silicon nitride, but in the 5th batch et seq., precipitation ofsilicon oxide took place, and it was impossible to continue theoperation.

Comparative Example 4

Etching of silicon nitride was carried out under the same conditions asin Example 5 by using a composition wherein in the composition inExample 5, 0.02% of fluoroboric acid was changed to 1% of fluoroboricacid, and further 0.1% of nitric acid was added.

1.5% of fluoroboric acid was added in an amount of 4% to every batch,and in the first batch, the SiN etching rate was 45.4 nm/min, and theetching rate of the thermally oxidized film was 84.6 nm/min. In the 12thbatch, the SiN etching rate was 49.0 nm/min, and the etching rate of thethermally oxidized film was 70.1 nm/min.

The etching selectivity for silicon nitride to silicon oxide (siliconnitride/silicon oxide) in each batch was examined, whereby theselectivity was very low at a level of 0.5 in the first batch and 0.7 inthe 12th batch.

The entire disclosures of Japanese Patent Application No. 2007-048606filed on Feb. 28, 2007, Japanese Patent Application No. 2007-157973filed on Jun. 14, 2007 and Japanese Patent Application No. 2007-330109filed on Dec. 21, 2007 including specifications, claims, drawings andsummaries are incorporated herein by reference in their entireties.

1. An etching method for silicon nitride, which comprises etchingsilicon nitride with a composition comprising a phosphorus compoundand/or a fluoride thereof, a boron compound and/or a fluoride thereof, asilicon compound and/or a fluoride thereof, and water.
 2. The etchingmethod for silicon nitride according to claim 1, wherein the phosphoruscompound and/or a fluoride thereof is at least one member selected fromthe group consisting of phosphoric acid, phosphorus fluoride, phosphorustrifluoride, phosphorus pentafluoride, fluorophosphoric acid and afluorophosphate.
 3. The etching method for silicon nitride according toclaim 1, wherein the boron compound and/or a fluoride thereof is atleast one member selected from the group consisting of boric acid, aborate, boron fluoride, boron trifluoride, fluoroboric acid and afluoroborate.
 4. The etching method for silicon nitride according toclaim 1, wherein the silicon compound and/or a fluoride thereof is atleast one member selected from the group consisting of a silicon halide,hexafluorosilicic acid, a hexafluorosilicate, an alkoxysilane, analkylsilane, silicic acid, a silicate, silicon fluoride, siliconchloride, ammonium hexafluorosilicate, tetraethoxysilane,tetramethoxysilane and methyltrimethoxysilane.
 5. The etching method forsilicon nitride according to claim 1, wherein the boron content in thecomposition is from 0.001 to 10 wt %.
 6. The etching method for siliconnitride according to claim 1, wherein the silicon content in thecomposition is from 0.001 to 0.01 wt %.
 7. The etching method forsilicon nitride according to claim 1, wherein the fluorine content inthe composition is from 0.010 to 0.050 wt %.
 8. The etching method forsilicon nitride according to claim 1, wherein the composition furthercontains nitric acid and/or a nitrate.
 9. The etching method for siliconnitride according to claim 1, wherein etching is carried out at atemperature of from 120° C. to 180° C.
 10. The etching method forsilicon nitride according to claim 1, wherein at least one memberselected from the group consisting of a phosphorus compound and/or afluoride thereof, a boron compound and/or a fluoride thereof, a siliconcompound and/or a fluoride thereof, nitric acid and/or a nitrate, andwater, is additionally added.
 11. An etching composition for siliconnitride as defined in claim 1.